Lignocellulosic biomass including wooden biomass consists of about 20% of hemicellulose, about 50% of cellulose, and about 30% of lignin. Hemicellulose and cellulose are decomposed to saccharides by a saccharification treatment, and then the saccharides are fermented with a fermentation microorganism such as yeast, and thus ethanol can be produced. Saccharification of hemicellulose gives C5 saccharides and C6 saccharides, and saccharification of cellulose gives C6 saccharides.
The term C5 saccharides used herein refers to pentoses such as xylose or arabinose, and oligosaccharides thereof. The term C6 saccharides used herein refer to hexoses such as glucose or galactose, and oligosaccharides thereof.
In alcoholic fermentation of a biomass saccharified solution, yeast is principally used, and alcoholic fermentation is conducted in a fermentation tank for about 8 to 72 hours. However, since alcoholic fermentation occurs in a condition that bacterial contaminants (for example, Leuconostoc, Citrobacter or Lactococcus bacteria) are easy to proliferate, the saccharides will be consumed by bacterial contaminants when the saccharified solution is contaminated with the bacterial contaminants.
Further, bacterial contaminants include those inhibiting growth of yeast, and those inhibiting alcoholic fermentation with yeast. Therefore, if bacterial contaminants proliferate in the saccharified solution in the fermentation tank, ethanol yield decreases, and the economical practicability of bioethanol production can be impaired.
Patent Literature 1 discloses a continuous culture and fermentation device for ethanol-producing microorganism enabling perfectly continuous supply of a substrate and capable of continuous culture and fermentation of a specified ethanol-producing microorganism without leading deterioration in the productivity. In Patent Literature 1, a fermented solution (saccharified solution) is drawn out from the fermentation tank and solid-liquid separated, and the ethanol-producing microorganism is recovered into the fermentation tank. It is disclosed that penicillin, which is a β-lactam antibiotic, is preferably added to the fermented solution.
On the other hand, as an alcoholic fermentation method of a saccharified solution that does not involve an antibiotic, a method of suppressing proliferation of bacterial contaminants by adding acid also receives attention. Patent Literature 2 discloses yeast of Candida glabrata (NFRI3164 strain) capable of growing in the presence of 5% lactic acid as novel yeast having acid tolerance suited for alcoholic fermentation in the condition that proliferation of bacteria is suppressed by addition of acid.